This invention relates primarily to interferometers and, more particularly, to an interferometer sensor or transducer which utilizes fiber optics transmission elements.
The primary function of an interferometer is for analyzing or filtering the spectral content of a beam of light, and it is known that an interferometric measurement is altered when one or two beams of light are used and one of the beams is affected in some way. Interferometers employing a pair of fiber optic elements for providing its two light paths are known. In that arrangement, physical interaction with one of the fiber elements (the other fiber being maintained as a reference) produces either a difference in path length for the two transmitted beams or a difference in the intensities thereof, or both. A measurement of the differences indicates the magnitude of the physical interaction.
Fiber optics devices are well-known to comprise plastic or glass structures having a core of relatively high index of refraction surrounded by a cladding of relatively low index of refraction. The outstanding feature of fiber optics devices is their ability to guide light by means of the core, even as the fiber device is bent or curved. Thus, the art and science of fiber optics offers a particular opportunity in terms of the usage and value of interferometric measurement.
Optical fibers, both multimode and single mode, are well known. In the single mode fiber, its single mode is essentially transmitted by having a fiber core of a diameter which allows only limited mode transmission in terms of wavelengths greater than a specified "cut-off" wavelength. However, a single mode fiber, which is circularly symmetric, actually has two possible polarization states or modes, both having the same propagation constant. Thus, a so-called "single mode" fiber in effect has two modes or optical paths which degenerate in propagation constant. On the other hand, a fiber which is not circularly symmetric removes the degeneracy such that the two polarization states have different propagation constants. In any event, such "single mode" fibers are still commonly referred to as "single mode" without regard to the two polarization states included.